I do understand where the error is about the missing number: Missing number, treated as zero. \end{tabulary}
\documentclass[a4paper,11pt]{article}
\usepackage{hyperref}
\usepackage{fontspec}
\usepackage[english, ngerman]{babel}
\usepackage{soul} % for strikethrought
\usepackage{xcolor} % for highlighting
\usepackage{booktabs} % for rules within a table
\usepackage{tabulary} % for tables with long lines
\begin{document}
\subsection{Basic HTML Table}
\begin{table}[ht]
\centering
\begin{tabulary}{\linewidth}{ C C C }
\toprule
\textbf{Firstname} & \textbf{Lastname} & \textbf{Age} \\
\midrule
Jill & Smith & 50 \\
\midrule
Eve & Jackson & 94 \\
\midrule
John & Doe & 80 \\
\midrule
\bottomrule
\end{tabulary}
\end{table}
%second table
Table with mulitple cols \hl{in the header}:
\begin{table}[ht]
\centering
\begin{tabulary}{\linewidth}{ C C C }
\toprule
\textbf{Name} & \multicolumn{2}{ C }{\textbf{Telephone}} \\
\midrule
Bill Gates & 55577854 & 55577855 \\
\midrule
\bottomrule
\end{tabulary}
\end{table}
\end{document}
Any idea? And if I drop the first table the whole thing gets even worse cause it reports Undefined control sequence. \end{tabulary}

tabulary. Anyway, you needcin the argument to\multicolumnand notC. – egreg Jan 11 '19 at 13:09